Doped silica glasses, including titania-doped silica glass, formulated to exhibit properties such as low thermal expansion and low variations in thermal expansion are typically formed by vapor phase hydrolysis/oxidation processes in which the doped materials are deposited as soot particles into a revolving collection cup in a refractory furnace. The glass material formed in this way and referred to as a “boule” tends to acquire periodic inhomogeneities in the direction of deposition. The periodic inhomogeneities in the boules, referred to as “striae”, are generally layered in planes that extend normal to the boule axes from the bottom to the top of the boules.
Blanks can be cut out of the boules for forming various optics, such as mirrors, and other components. For many applications, including lithographic elements for conveying soft x-ray or extreme ultraviolet radiation, the doped silica glass must be qualified or otherwise characterized in accordance with certain critical criteria, which can include measurements of the coefficient of thermal expansion and crossover temperature.
Strip samples are cut from the boules for testing. The strip samples are typically cut along parallel, generally radially oriented lines, from the top to the bottom of the boules. The strip samples include parallel front and back faces cut through the depth of the boule and top and bottom faces corresponding to portions of the top and bottom surfaces of the boule. The two end faces of the strip sample correspond to the inner and outer end cuts through the boule.
The strip samples are generally measured using ultrasonic pulse echo techniques that measure variations in the speed of sound through the front and back faces of the samples. The strip samples must be quite thick to obtain useful measurements and spatial resolution of the measurements is limited by the wavelengths of the sound waves and physical limitations of the acoustic probe.
Interferometric techniques have been used for detecting the striae in boule samples, but the techniques are limited by both the index variation and spacing of the striae. While useful for some purposes, the characterization of the striae only is generally insufficient to qualify the glass material for use as an ultra-low thermal expansion material. In addition, diffusion/diffraction effects produced by finer periods and index variations associated with the striae of many doped silica materials prohibit the use of this technique.